1. Field of the Invention
The present invention relates to scatterometry. In particular, it relates to far field measurements involving the simultaneous measurement of reflected or transmitted scattered wavefronts through the use of hemispherical and/or spherical light-scatter and phase-measuring technology.
2. Description of the Related Art
Scatterometers are used to analyze light sources and material properties by measuring how a particular material or surface reflects or transmits light in spherical (“scattered”) radiation. If the surface is not radiating its own light, such as in the case of an LED, a light source, such as a laser, can be directed at some angle onto the surface to produce scattered light from the point of incidence. If the surface is specular or otherwise radiates the incident light in a unidirectional fashion, all the light will be directed away from the surface along a single beam. Otherwise, the reflected light will be scattered and radiated throughout the hemisphere above the test surface. The science and mathematics of scattered light are well developed. See, for example, the book by J. C. Stover, “Optical Scattering: Measurement and Analysis,” McGraw-Hill, N.Y. (1990).
A typical scatterometer includes a laser shining a beam on a test surface and a single detector that is mechanically scanned over a 180-degree circular arc around the illuminated spot. The detector's field of view is kept on the illuminated spot regardless of the viewing angle. At each view angle, a measurement is made of the light intensity, thereby generating a spatial distribution of the scattered light. The distribution over the scan arc is dependent on the characteristics of the test surface, including the type of material, surface roughness, reflectivity, color, surface structure, sub-surface damage, and others.
Scanning a single detector has greatly limited the ability to measure scattering over large areas, such as over an entire hemisphere, and to measure large dynamic ranges of intensity. Moreover, scanning over an entire hemisphere takes a lot of time. Thus, measuring scattering over many spots on the test surface has been quite impractical with a single detector. Measuring dynamic events at multiple measurement positions has also not been possible with this approach.
In order to overcome these limitations, multiple detectors with a single detecting element distributed over a larger hemispherical area have been used. For example, a limited number of detectors (from 10 to 120 detectors) have been placed along an arc, where the arc center is the measurement point. This approach works well when the scatter field (or far-field pattern, as defined herein) is uniform, but it does not work when the far field pattern is random or has a high frequency content.
Another prior-art approach is to shine a laser beam through a hole in a translucent dome onto a test sample positioned at the center of the dome. The scattered light from the test sample illuminates the interior dome surface, which, because it is translucent, permits the use of a camera to view the scattered light from the dome's exterior face. This approach works satisfactorily for some basic applications. However, the light can also scatter laterally between the dome's interior and exterior surfaces producing corrupted measurement results. This method also does not allow for easy measurement of the light scattered at angles approaching 90 degrees from the direction normal to the surface of the camera (i.e., the hemispherical dome's edge).
Another method for hemispherical scattered-light measurements is described in U.S. Pat. No. 5,313,542, U.S. Pat. No. 5,475,617, U.S. Pat. No. 5,615,294, U.S. Pat. No. 5,640,246, and U.S. Pat. No. 5,729,640. This approach is based on the use of a fiber-optic bundle to measure a portion of the hemisphere in three dimensions. This is done by cutting a spherical surface onto a tapered portion of a fiber bundle. The opposite end of the fiber bundle is tapered to a much smaller size for coupling to a camera. This method allows very high-resolution measurements over a portion of the hemisphere. However, its main disadvantages are a lower dynamic range, the expense of the fiber bundles, the loss of phase information as the light travels though the fiber, and the mechanical scanning requirements for full hemispherical measurements.
Thus, all of these prior-art methods have common limitations in the degree of far-field coverage and angular measurement resolution. In addition, they cannot measure phase. Therefore, a system capable of measuring both spherical scatter and phase with a single instrument in real-time would be very desirable in the art and would provide unprecedented medical diagnostic capability. The present invention provides a solution to many of the problems of the prior-art devices and enables the acquisition of significantly more measurement data at speeds not possible before. Moreover, both spherical and phase information from scattered light can be acquired simultaneously.